Electrochemical cell

ABSTRACT

A flow-frame for forming a subassembly; said sub-assembly comprising a bipolar electrode and an ion-selective membrane mounted on said flow-frame and wherein said sub-assembly may be stacked together with other such subassemblies to create an array of electrochemical cells; wherein said flow-frame is formed from an electrically insulating material and comprises at least four manifold-defining portions which also define pathways for the passage of the anolyte/catholyte. Such pathway may define a labyrinthine path which may be spiral in shape between the manifold and the chamber entry/exit port.

The present invention relates to electrochemical systems for the storageand delivery of electrical energy and, in particular, to apparatus forbuilding such systems.

BACKGROUND OF THE INVENTION

Industrial electrochemical systems, such as secondary batteries, fuelcells and electrolysers, typically consist of modules which eachcomprise a number of repeating layered sub-assemblies clamped togetherto form a stack. For example, in a secondary battery of the redox flowtype each sub-assembly typically consists of an electrically insulatingflow-frame (i.e. a device which supports the other constituent parts ofthe sub-assembly and which also defines channels for the flow ofelectrolytes), a bipolar electrode, an ion-selective membrane or acombined membrane-electrode material and, optionally, other componentlayers such as meshes or electrocatalytic materials. A plurality of suchsub-assemblies may be sandwiched together between suitable end-plates soas to create a plurality of electrochemical cells in series. Each cellthus comprises the positive and negative surfaces of two bipolarelectrodes with an ion-selective membrane positioned therebetween so asto define separate anolyte-containing and catholyte-containing chamberswithin each cell, said chambers optionally comprising additionalcomponents such as meshes or electrocatalytic materials. The twoelectrolytes are typically supplied from two reservoirs to the cellchambers via an electrolyte circulation network. Electrochemical systemsof this type are well known to a person skilled in the art.

In the manufacture of components for the creation of such assembliesthere are a number of important considerations. In particular, it isdesirable to suppress shunt currents within the electrolyte circulationnetworks. Shunt currents occur because of the conductive pathways thatare created by the network of electrolyte connections linking the cellchambers. They are a particular problem for stacks which contain a largenumber of bipoles and their occurrence decreases the efficiency of thecell. Additionally, it is advantageous to make efficient use of all theavailable surface area of the electrode. In order to do this theelectrolytes must be distributed evenly over the surfaces of theelectrodes upon entering the cell chambers. Furthermore, in order toensure that the fluids which are inside the stack are isolated from eachother and contained successfully with minimal leakage to the outside, itis necessary for satisfactory seals to be provided between theindividual components within the stack.

The occurrence of shunt currents within such cell arrays is discussed byP. G. Grimes and R. J. Bellows in a paper entitled “Shunt currentcontrol methods in electrochemical systems-applications”, appearing inElectrochemical Cell Design, R. E. White, Ed.: Plenum Publishing Corp,1984, page 259. Typically, shunt currents are reduced by the provisionof labyrinthine pathways for the electrolytes between the electrolytecirculation networks and the individual cell chambers. One method forachieving such a pathway has been to connect long-tubes between theelectrolyte circulation networks and each of the individual cellchambers. However this method suffers from the disadvantage that itrequires at least two seals, one at either end of the tube, whichcomplicates the assembly procedure and can cause problems withelectrolyte leakage especially since the seals must cope with pressuredifferentials which usually exist between the internal system and theexternal environment. Another method for providing a labyrinthinepathway involves forming a long groove into the surface of theflow-frame from a point in communication with the electrolytecirculation network to a point in communication with the individual cellchamber. On stacking the sub-assemblies a plate is sandwiched betweensuccessive layers so as to seal the groove and form a labyrinthinepathway for the electrolyte. This method suffers from the disadvantagethat the costs of forming the grooves can be high and an extra layer,i.e. the plate, must usually be incorporated into the assembly toprovide efficient sealing. This method also often requires large frameareas upon which to form the grooves. Electrolyte leakage is aparticular problem in methods for controlling shunt currents whichinvolve labyrinthine pathways for the electrolytes. Efficient fluidicsealing of the pathways is required to prevent leakage and this problemmay be exacerbated by the fact that high pumping pressures are oftenrequired to push the electrolytes through the narrow pathways. Othersolutions to the problem of shunt currents include electrically breakingthe circuit by arranging for the flow to break up into droplets or sprayor by using some form of syphon; even mechanical water wheel typestructures have been proposed. Such solutions are rarely used inpractice however because the mechanical and flow regimes are difficultto implement. Other solutions, rather than eliminate the shunt currents,attempt to control their effects, for example, by deliberately shuntingthe current through an auxiliary electronic circuit or by passing anappropriate current through the common manifold or channelinterconnectors. However, these techniques do not necessarily reduceoverall power loss.

It would be advantageous to provide a flow-frame, suitable for forming asub-assembly as described above, which is a repeating structural unitwithin an array of electrochemical cells formed from a stack of saidsub-assemblies. The flow-frame would advantageously provide a frameworkfor supporting all the other elements of the cell array within a sealedenvironment together with means for providing resistance to shuntcurrents and means for distributing an even flow of electrolyte throughthe chambers of each cell.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a flow-frame for forming asub-assembly; said sub-assembly comprising a bipolar electrode and anion-selective membrane mounted on said flow-frame and wherein saidsub-assembly may be stacked together with other such sub-assemblies tocreate an array of electrochemical cells, each cell thus comprising twoelectrode surfaces with an ion-selective membrane positionedtherebetween so as to define separate anolyte-containing andcatholyte-containing chambers within each cell; wherein said flow-frameis formed from an electrically insulating material and comprises

(i) a chamber-defining portion for supporting an electrode and amembrane within a defined space,

(ii) at least four manifold-defining portions which, on stacking saidflow-frames, define four manifolds through which the anolyte and thecatholyte are supplied to and removed from said anolyte-containing andcatholyte-containing chambers,

(iii) at least two chamber entry ports for allowing the anolyte and thecatholyte to flow from said manifolds into said anolyte-containing andcatholyte-containing chambers, and

(iv) at least two chamber exit ports for allowing the anolyte and thecatholyte to flow from said anolyte-containing and catholyte-containingchambers into said manifolds,

characterised in that one or more of the manifold-defining portions alsodefine a pathway for the passage of the anolyte/catholyte between themanifold and the chamber entry/exit port.

Thus, in the present invention, the pathway for the passage of theanolyte/catholyte between the manifolds and the chamber entry/exit portsis formed within the manifold-defining portions of the flow-frame. Thepathway may comprise grooves cut into one surface of themanifold-defining portions of the flow-frame. On stacking theflow-frames the grooves are sealed by the flat surface of themanifold-defining portion of the adjacent frame to form sealed pathways.Preferably the pathway defined within the manifold-defining portionsdoes not allow electrolyte to travel in a straight line directly betweenthe manifold and the chamber entry/exit ports. Preferably it causes theelectrolyte to take a tortuous or labyrinthine path between the manifoldand the chamber entry/exit ports.

The pathway is advantageously incorporated into the manifold-definingportions because pressure differentials which would drive leaks are keptrelatively small, reducing the problems associated with the requirementfor efficient fluidic sealing of said pathway. Furthermore, because thepathway is formed within the manifold-defining portions any leakagecaused by inefficient sealing is contained within the manifold and doesnot contaminate other parts of the assembled module. Thus the flow-frameof the present invention is more tolerant to leakage than flow-framesknown in the art.

Preferably the pathway is substantially spiral in shape. A substantiallyspiral pathway is preferred for a number of reasons. Firstly, such apathway avoids sudden drops in fluid pressure which can be caused by thepresence of sharp corners in the pathway. Secondly, it achieves the aimof separating fluids at different electrical potentials whilst occupyingthe minimum space possible. Thirdly, it maintains a near-circularmanifold cross-section which is ideal for the efficient flow ofelectrolytes within the manifold. Finally, it is relatively easy tomanufacture.

Preferably, the manifold-defining portions are themselves distanced fromthe main chamber-defining portion. This further reduces the risks ofshunt currents travelling between the chambers and the manifolds.

In a preferred embodiment of the present invention the pathway isdefined by a part which is releasably. insertable within saidmanifold-defining portions. As indicated above the magnitude of theproblems associated with shunt currents depends upon the number ofbipolar electrodes which make up the complete stack. The greater thenumber of bipoles the more serious are the losses caused by theoccurrence of shunt currents. It is further known that the problemsassociated with shunt currents also vary according to the nature of theelectrolytes in a given system, the position and performance of anindividual sub-assembly within the stack itself and the nature of thearray of stacks. An advantage of the provision of said releasablyinsertable parts is that it allows the customisation of individualflow-frames within each sub-assembly to adjust the resistance to shuntcurrents and simultaneously to adjust the resistance to electrolyte flowin the manifold depending upon the position of the sub-assembly withinthe stack and also depending upon the size and nature of the stack as awhole. Furthermore, it allows customisation of the individualmanifold-defining portions within each flow-frame depending upon theidentity of the electrolyte within the manifold and whether it is beingsupplied to, or removed from, the cell chamber. A further advantageassociated with the provision of releasably insertable parts is thatwhen the sub-assemblies are stacked to form a cell array the releasablyinsertable parts may be staggered relative to one another so that thepoints within the manifold from which electrolyte is drawn into thepathways are distanced from one another so as to further reduce theeffects of shunt currents.

Around the perimeter of the chamber-defining portion of the flow frameit is necessary that the surface topography thereof remainssubstantially continuous so that when a membrane is included in thelayered sub-assembly an efficient seal is formed to ensure that theanolyte and catholyte chambers remain substantially isolated. Suchisolation may be achieved with an elastomeric seal, a weld, or by othermeans. In a preferred embodiment, extending around the perimeter of onesurface of the chamber-defining portion of the frame and within theintegral sealing means described below is provided a small,substantially continuous, raised portion so that, on stacking theframes, a mechanical pinch is formed between said raised portion on oneframe and a flat or grooved surface on the chamber-defining portion ofthe adjacent frame in the stack. The mechanical pinch is designed tosecure a membrane in position when it is included as a part of thesub-assembly in order to limit cross-contamination of electrolytes atthe edge of the membrane. The advantages of using a mechanical pinch asdescribed above are that it is relatively easily manufactured as part ofthe frame and it achieves a sufficiently tight grip to isolate theanolyte and catholyte given that they tend to have only modest pressuredifferentials. In a preferred embodiment such a mechanical pinch mayalso be created between the grooves which are cut into themanifold-defining portions by providing a small substantially continuousraised portion between said grooves. In this case the pinch ensures thatwhen the flow-frames are stacked the grooves which create thelabyrinthine pathway are isolated from one another so that fluid andcurrent cannot flow between adjacent grooves.

The flow of the electrolytes from the manifolds to the electrolytechambers, and vice versa, must be effected whilst maintaining the mutualisolation of the anolyte and catholyte containing chambers. Theelectrolytes enter and exit the anolyte and catholyte chambers by meansof the chamber entry/exit ports. In flow-frames known in the art thesecommonly take the form of flow channels situated entirely within theframe thickness. However this type of channel is difficult to machine ormould. Accordingly, in a preferred embodiment of the present inventionone or more of the chamber entry/exit ports comprise optionallyreleasable inserts shaped so that on insertion into the flow-frame theyform flow channels between the end of the pathway defined by themanifold-defining portions and the anolyte/catholyte containingchambers. The outer surface of said insert is preferably shaped so thaton placement of the insert within the chamber entry/exit ports thesurface topography of the chamber-defining portion of the flow-frameremains continuous in the vicinity of the chamber entry/exit ports. Thisis advantageous because, as mentioned above, it enables a sufficientlytight seal to be formed between successive sub-assemblies upon stackingand ensures that the membrane layer is tightly gripped betweensuccessive flow-frames so as to substantially isolate theanolyte-containing and catholyte-containing chambers from one another.The opposite, inner surface of the insert which contacts the floor ofthe chamber entry/exit port has one or more grooves cut into thesurface, the size and shape of the grooves being determined by thedesired flow characteristics for the insert. Preferably the grooves aredesigned so as to direct the flow of anolyte/catholyte evenly over thesurfaces of the electrodes. The flow characteristics desired for aparticular chamber entry/exit port within a flow-frame will depend upona number of factors including the overall size of the stack, theposition of the flow-frame within the stack and the flow properties ofthe electrolytes in question. The inserts can be customised accordingly.Preferably, the inserts are releasably inserted into place so that theflow characteristics of the cell entry/exit ports for a particularflow-frame can be altered simply by inserting a different shaped insertrather than redesigning the entire flow-frame. A further advantage ofthis design is that the inserts are relatively easy to manufacture andit avoids the need to machine flow channels through the thickness of theflow-frame.

In addition to the provision of releasable inserts within the chamberentry/exit ports, the distribution of the electrolytes over the surfacesof the electrode may be further improved by the inclusion ofappropriately sculpted flow distribution means extending oversubstantially the entire width of both ends of the flow-frame andlocated at a point adjacent to the chamber entry/exit ports. Upon theformation and stacking of electrode/membrane/frame sub-assemblies theflow distribution means, together with the membrane, define channels forthe flow of the electrolytes along the width of either end of the frameand apertures opening into the cell chambers on either side of theelectrode for the flow of the electrolytes onto or away from theelectrode surfaces. The resistance to fluid flow across the width of theends of the flow-frame is determined by the cross-sectional area of thechannels whilst the resistance to fluid flow onto the surface of theelectrode is determined by the size of the aperture opening into thecell chambers. Together, the cross-sectional area of the channels andthe size of the aperture act so as to spread the flow of theelectrolytes evenly over the surfaces of the electrode. Thus, in apreferred embodiment of the present invention, the resistance to flowacross the width of the flow-frame is lowest at the points closest tothe chamber entry/exit ports by provision of a channel with a largecross-sectional area and highest at the points furthest from the chamberentry/exit ports by provision of a channel with a low cross-sectionalarea. The size of the aperture into the cell chambers remains constantacross the width of the flow-frame. Thus, at a point close to thechamber entry/exit ports the electrolytes flow easily along the width ofthe frame either spreading out over the width of the frame or beingdrawn in from across the width of the frame. In contrast, at a pointfurther from the entry/exit ports the electrolytes flow less easilyalong the width of the flow frame and are therefore directed towards ordrawn from the electrode surfaces. Thus, the electrolytes are suppliedto and removed from the electrode surfaces with a more even flow overthe entire width of the electrode. In the present invention, thevariations in resistance to fluid flow apply simultaneously and in anopposite fashion at either end of the frame.

Preferably the entire perimeter of the flow-frame is provided with meansfor forming a seal between adjacent frames when they are stacked to forma sub-assembly. More preferably said sealing means comprises an integralsealing means as described in our co-pending application numberWO97/24778. The integral sealing means comprises a continuous groove onone face of the frame which defines a female opening having a width of wand a depth of h and a continuous upstand on the other face of the framehaving a width of >w and a height of <h . The sealing means is designedto hold the frames together when they are formed into a stack and toprevent the escape of the electrolytes from the cells.

Preferably, extending inwardly from the chamber-defining portion of theframe, is provided means for supporting an electrode within the spacedefined by said chamber-defining portion.

The flow-frame of the present invention may be formed from anyelectrically insulating material. Preferably however it may be formedfrom one or more polymers selected from polyethylene, polypropylene andcopolymer blends of ethylene and propylene, acetal, nylons, polystyrene,polyethylene terephthalate, polyvinylidene fluoride, polyvinyl chloride,polytetrafluoroethylene, fluorinated ethylenepropylene copolymer,polyfluoramide, chlorinated polyoxymethylene and many others. Thedesired configuration for the flow-frame may be formed from thesepolymeric materials by machining, injection moulding, compressionmoulding or extrusion.

The present invention also includes within its scope an electrochemicalapparatus comprising a flow-frame as hereinbefore described.

The present application also includes within its scope anelectrochemical apparatus comprising a plurality of flow-frames, andeither a plurality of bipolar electrodes and a plurality ofion-selective membranes or a plurality of combined membrane-electrodematerials and, optionally, a plurality of meshes and/or electrocatalyticmaterials sandwiched together so as to create an array ofelectrochemical cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the drawings in which:

FIG. 1 is a schematic representation of a flow-frame according to thepresent invention.

FIG. 2 is a magnified representation of portion X of FIG. 1 showing, indetail, a manifold-defining portion of a flow-frame according to thepresent invention, including the optionally releasable spiral pathwaydefining parts but not the optionally releasable inserts.

FIGS. 3A, 3B and 3C show cross-sectional views along lines A—A, B—B andC—C respectively.

FIG. 4 is a representation of a releasable insert according to thepresent invention.

FIG. 5 is an exploded view of a stack of sub-assemblies, eachsub-assembly being formed from a flow-frame according to the presentinvention, a bipolar electrode, an optional mesh or catalytic layer anda membrane. Such a stack may form part of an array of electrochemicalcells.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the flow-frame comprises a substantiallyrectangular chamber-defining portion 1 with four substantially circularmanifold-defining portions 2,3,4 and 5 positioned two at each end of therectangular chamber-defining portion. The chamber-defining portionserves to support a bipolar electrode and a membrane within the spacecreated therein. The frame/electrode/membrane sub-assembly thus formedmay sandwiched together with a plurality of other such sub-assemblies soas to create a plurality of electrochemical cell in series (see FIG. 5).Each cell thus comprises the positive and negative surfaces of twobipolar electrodes with a membrane positioned therebetween so as todefine separate anolyte-containing and catholyte-containing chamberswithin each cell. It will be understood by those skilled in the art thatthe positioning of the manifold-defining portions relative to thechamber-defining portion and the chosen rectangular and circular shapesof the frame and manifold-defining portions respectively are notcritical to the present invention. In the illustrated embodiment themanifold-defining portions 2 and 4 define, upon stacking the frames,manifolds 6 and 7 which may supply/remove the catholyte to/from thecatholyte-containing chambers. The other manifold-defining portions 3and 5 define, upon stacking the frames, manifolds 8 and 9 which maysupply/remove the anolyte to/from the anolyte-containing chambers.

Referring to FIG. 2 and FIG. 3A, the manifold-defining portions 2 and 3(only 3 is shown in FIG. 2 but the same structural features are alsopresent in 2, 4 and 5) contain optionally releasable ring-shaped members10 and 11 (only 11 is shown in FIG. 2) which are shaped so as to providea tight fit within the manifold-defining portions 2 and 3. Although theillustrated embodiment of the present invention provides a tight fit forretaining the ring-shaped members 10 and 11 within the manifold-definingportions 2 and 3 other means for locating and securing the ring-shapedmembers in position are envisaged and are included within the scope ofthe invention. The releasable ring-shaped members 10 and 11 comprise twoparallel surfaces 12 and 13, one of which is a substantially flatsurface and the other of which comprises a spiral groove 14 cut therein.On stacking the frames, the groove 14 is sealed by the coming togetherof flat surface 13 of one frame with flat surface 12 of the adjacentframe so as to define an extended spiral pathway for the passage of theanolyte/catholyte between the manifold 8 and the chamber entry/exit port15. Surface 16 delineates the circumferential face of the optionallyreleasable members which provides a tight fit with the inner face of themanifold-defining portions. The optionally releasable members may beremoved and replaced by a member with a different groove length ordifferent groove cross-sectional area as required. Attention is drawn tothe relative orientation of the optionally releasable members 10 and 11in FIG. 3A of the illustrated embodiment. It will be noted that thegrooves are present on opposite faces of the resultant flow frame. Thusthe two chamber entry/exit ports which form part of the twomanifold-defining portions at one end of the frame supplyanolyte/catholyte to opposite faces of an electrode when said electrodeis mounted within the rectangular space defined by the chamber-definingportion.

Referring to FIG. 2 and FIG. 3B, adjacent to the perimeter of theflow-frame and extending all the way around the perimeter are means 17and 18 for forming a seal between successive frames when they arestacked to form an array of electrochemical cells. The means comprise acontinuous groove 17 which defines a female opening having a width of wand a depth of h and a continuous upstand 18 having a width of >w and aheight of <h.

Referring to FIG. 2 and FIG. 3C, at each end of the rectangularchamber-defining portion, at a point adjacent to the chamber entry/exitports, there are provided sculpted portions 19 and 19′ extending oversubstantially the entire width of the chamber-defining portion whichdefine channels 20, 20′, 21 and 21′ on either side of the sculptedportions 19 and 19′. The cross-sectional areas Y, Y′, Z and Z′ of thechannels 20, 20′, 21 and 21′ respectively vary along the length of thesculpted portions, and do so in an opposite fashion at either end of thechamber-defining portion. That is, Y is large when Y′ is small and viceversa, whilst Z is large when Z, is small and vice versa. Thecross-sectional areas are larger at points close to the chamberentry/exit ports and smaller at points further from the chamberentry/exit ports.

Referring to FIG. 2 and FIGS. 3B and 3C, extending inwardly from thesculpted portions 19 and 19′ at each end of the frame and from the innerfaces 22 of the sides of the frame, there is provided a continuous lip23 to which an electrode (not shown) may be attached on forming asub-assembly.

Referring to FIG. 2 and FIGS. 3B and 3C, extending around the perimeterof one surface of the rectangular chamber-defining portion of the frame,inside the means 17 and 18 for forming a seal between successive frames,is provided a small, substantially continuous, raised portion 24 forforming a mechanical pinch, on stacking the frames, between the raisedportion 24 on one frame and the flat surface 25 of the chamber-definingportion of the adjacent frame in the stack. This mechanical pinch isdesigned to secure the membrane in position when it is included as partof the sub-assembly and to minimise the crossover and/or mixing ofelectrolytes. The continuity of this raised portion is maintained in theregion of the chamber entry/exit ports by an identical raised portion onthe surface 33 of the inserts as described below.

Extending from the outer edge of the flow-frame there is provided a lip26 which aids handling of the flow-frame on assembling and disassemblingstacks of sub-assemblies.

Referring to FIG. 4, the insert for the chamber entry/exit portscomprises a body 30 which is shaped so as to provide a tight fit withinthe chamber entry/exit ports. The surface 31 of the inserts whichcontacts the floor of the chamber entry/exit ports is provided with aplurality of grooves 32, in this case four, for the passage of theelectrolyte. The opposite surface 33 of the insert is shaped so that onplacement of the insert within the chamber entry/exit ports the surfacetopography of the rectangular portion of the frame, including the raisedportion 24 remains continuous in the vicinity of the chamber entry/exitports. The insert of the illustrated embodiment also possesses threeshaped projections 34, 35 and 36 which extend from the body 30 of theinsert into the regions containing the sculpted portions 19 and 19′.These projections are shaped so as to distribute the flow of electrolyteevenly over the surfaces of the electrode. The design of the releasableinserts can be altered so as to provide different flow characteristicsfor the flow frame which can thus be customised according to thecharacteristics desired.

Referring to FIG. 5, there is shown an exploded view of a small stackcomprising four sub-assemblies. The first two sub-assemblies and theflow-frame for the third sub-assembly are shown clamped together. Of thefirst two sub-assemblies, only the flow-frames (40 and 41) are visible.The third and fourth sub-assemblies are exploded to show the constituentlayers thereof. The flow-frame 42 for the third sub-assembly supports abipolar electrode 43 within the space defined by said flow-frame 42. Thebipolar electrode may optionally be surfaced on one side with a layer 44which may be formed from a porous and/or electrocatalytic material.Although not illustrated, it will be appreciated by a person skilled inthe art that such a layer of porous and/or electrocatalytic material mayalternatively be used to surface the other side of the bipolarelectrode. Furthermore, two such layers may be used to surface bothsides of the bipolar electrode. The next layer in the sub-assembly isthe membrane 45. This layer is slightly larger in area than the bipolarelectrode 43 and optional layer 44. Components 42 to 45 make up thethird sub-assembly. Similarly, the fourth sub-assembly is made up of aflow-frame 46, a bipolar electrode 47, an optional layer 48 and amembrane 49. The stack may comprise many more than the foursub-assemblies shown in FIG. 5 and in an electrochemical cell comprisingsuch a stack, suitable end-plates (not shown) will be provided at eitherend of the stack.

What is claimed is:
 1. A flow-frame for forming a sub-assembly; saidsub-assembly comprising a bipolar electrode and an ion-selectivemembrane mounted on said flow-frame and wherein said sub-assembly may bestacked together with other such sub-assemblies to create an array ofelectrochemical cells, each cell thus comprising two electrode surfaceswith an ion-selective membrane positioned therebetweeen so as to defineseparate anolyte-containing and catholyte-containing chambers withineach cell; wherein said flow-frame is formed from an electricallyinsulating material and comprises (i) a chamber-defining portion forsupporting an electrode and a membrane within a defined space, (ii) atleast four manifold-defining portions which, on stacking saidsub-assemblies, define four manifolds through which the anolyte and thecatholyte are supplied to and removed from said anolyte-containing andcatholyte-containing chambers, (iii) at least two chamber entry portsfor allowing the anolyte and the catholyte to flow from said manifoldsinto said anolyte-containing and catholyte-containing chambers, and (iv)at least two chamber exit ports for allowing the anolyte and thecatholyte to flow from said anolyte-containing and catholyte-containingchambers into said manifolds, characterised in that one or more of themanifold-defining portions also define a pathway for the passage of theanolyte/catholyte between the manifold and the chamber entry/exit port.2. A flow-frame as claimed in claim 1 wherein the pathway comprises agroove cut into one surface of a manifold-defining portion of theflow-frame such that on stacking the flow-frames said groove is sealedby the flat surface of the manifold-defining portion of the adjacentframe to form a sealed pathway.
 3. A flow-frame as claimed in claim 1wherein the pathway defines a labyrinthine path between the manifold andthe chamber entry/exit port.
 4. A flow-frame as claimed in claim 1wherein the pathway is substantially spiral in shape.
 5. A flow-frame asclaimed in claim 1 wherein the pathway is defined by a part which isreleasably insertable within said manifold-defining portions.
 6. Aflow-frame as claimed in claim 1 wherein one or more of the chamberentry/exit ports comprise optionally releasable inserts shaped so as todirect the flow of anolyte/catholyte evenly over the surfaces of theelectrodes and maintain the surface topography of the chamber-definingportion of the flow-frame.
 7. A flow-frame as claimed in claim 1additionally comprising flow distribution means located adjacent to thechamber entry/exit ports which causes electrolyte to be spread evenlyover the surface of the electrode when the flow-frame forms part of asub-assembly.
 8. A flow-frame as claimed in claim 1 additionallycomprising sealing means extending around the perimeter of theflow-frame.
 9. A flow-frame as claimed in claim 1 additionallycomprising means for supporting an electrode within the space defined bythe chamber-defining portion.
 10. A flow-frame as claimed in clam 1additionally comprising means for forming a mechanical pinch between thechamber-defining portions of adjacent flow-frames when they are stackedto form an array.
 11. A flow-frame as claimed in claim 1 formed from apolymeric material.
 12. A flow-frame as claimed in claim 1 which isformed from one or more polymers selected from polyethylene,polypropylene and copolymer blends of ethylene and propylene, acetal,nylons, polystyrene, polyethylene terephthalate, polyvinylidenefluoride, polyvinyl chloride, polytetrafluoroethylene, fluorinatedethylene-propylene copolymer, polyfluoramide or chlorinatedpolyoxymethylene.
 13. A sub-assembly comprising a flow-frame as claimedin claim 1, a bipolar electrode and an ion-selective membrane.
 14. Anelectrochemical apparatus comprising a plurality of sub-assemblies asclaimed in claim
 13. 15. A sub-assembly as claimed in claim 13 whichfurther comprises a mesh layer and/or an electrocatalytic layer.
 16. Asub-assembly comprising a flow-frame as claimed in claim 1 and acombined membrane-electrode material.
 17. An electrochemical apparatuscomprising a plurality of sub-assemblies as claimed in claim
 16. 18. Asub-assembly as claimed in claim 16 which further comprises a mesh layerand/or an electrocatalytic layer.
 19. An electrochemical apparatuscomprising a flow-frame as claimed in claim
 1. 20. An electrochemicalapparatus comprising a plurality of flow-frames as claimed in claim 1sandwiched together so as to create an array of electrochemical cells.